It is desirable to make refrigerant evaporators efficient, compact, and lightweight. When compact and lightweight evaporators are used with air containing moisture, however, the moisture tends to condense on the evaporator as a layer of ice or frost. Before long, the ice clogs the evaporator and system efficiency is impaired.
The narrower air passages are between cooling coils or fins of an evaporator, the more quickly these passages accumulate ice and become obstructed. When the air passages are obstructed, airflow through the evaporator is impeded and efficiency of the refrigeration system incorporating the evaporator is also impaired.
In our previously issued patents and applications, it has been shown that tubing of an evaporator may serve as an electrical resistive heater, and that electrical current through this resistive heater may serve to melt and remove ice from the tubing and fins of the evaporator. We have used the term Pulse ElectroThermal Defrosting (PETD) to describe application of electrical power in pulses, typically of under a minute duration, and of high power density often greater than two kilowatts per square meter, to defrost evaporators and other devices.
In our prior work, electrical resistive heaters formed directly from common refrigeration tubing materials such as aluminum and copper have had low resistance. Providing reasonable electrical power to such low resistance resistive heaters requires heavy and expensive high current wiring and step-down transformers. For example, we have a system where the tubing of the evaporator itself serves as a secondary of a step-down transformer that is inductively coupled to a primary connected to an alternating current supply.
It is desirable to increase the electrical resistance of an evaporator to permit use of lower currents and higher voltages for melting and removing ice from tubing of the evaporator. Higher resistance has advantage in that it permits use of lighter wiring and less expensive switching devices and/or transformers.
We have also previously disclosed evaporators having higher resistance thin film resistive coatings over nonconductive or electrically insulated tubing. These embodiments are somewhat expensive to build because deposition of such thin film coatings is expensive.